Almost 1 MT/y of activated carbons are used in a very wide variety of applications(1). The largest volume use of activated carbons is in water treatment with other important applications including gas purification, decolourisation and adsorbency. More speciality, small volume, uses include catalysis, electrochemistry including fuel cells, biomedical devices, hydrogen storage, personal protection and automotive components. Activated carbons are used in different physical forms including powders, beads, cloths and monoliths.
Most activated carbons are made from highly abundant, low cost and essentially sustainable raw materials including coconut, coal, lignite, wood and fruitstones. The problems and limitations with such carbons include high levels of impurities, variable pore structures and limited physical forms in which they can be made stable. In particular, it would be desirable to make activated carbons with high mesoporosity(2-8) for liquid phase and biomedical applications, and to make such activated carbons which can be shaped into stable forms including monoliths for good control of pressure drop and good heat and mass transfer characteristics. Such materials can be used in more specialised applications and attract higher prices.
Solid acids are very important catalysts in the petroleum refinery and in the production of chemicals. At least 180 industrial processes using solid acids are in operation although many others still use conventional liquid and soluble acids that generally lead to health and safety, corrosion and separation problems. Solid acids are normally considered to be easier and safer to use, easier to separate and recover for reuse and lead to less process waste. They can also give more selective reactions due to pore constraints or other surface effects. A difficulty in the use of solid acids is that they become severely poisoned by water so that they are normally made anhydrous before use. Clearly the use of conventional solid acids in water as a solvent or co-solvent is normally not effective. However water stable or water tolerant solid acids would have great advantages in many cases, including the catalysis of reactions of water-soluble substrates, the catalysis of reactions of compounds derived from aqueous fermentation processes and generally in the avoidance of organic solvents in chemical processes. Great efforts have been made to develop water-tolerant solid acid catalysts but only with limited success, for example with some hydrophobic zeolites1. There is a great need to develop new water-tolerant but also active solid acid catalysts.
Similarly there is a need to develop new solid supported catalyst materials other than acid catalysts, such as basic materials or supported metals.
Conventionally, most activated carbons are made from highly abundant, low cost and essentially sustainable raw materials including coconut, coal, lignite, wood and fruitstones. The problems and limitations with such carbons include high levels of impurities, variable pore structures and limited physical forms in which they can be made stable. In particular, it would be desirable to make activated carbons with added functionality, such as including carbon acids, with high mesoporosity2-8 for example for liquid phase catalytic applications. It would also be desirable to be able to shape such materials into stable forms for good heat and mass transfer characteristics.
One known approach to the preparation of more mesoporous and shaped activated carbons is through the use of synthetic organic polymer precursors. However, these are not sustainable materials being largely derived from petroleum-sourced polymers.
Another approach, which is illustrated schematically in FIG. 1, for the preparation of mesoporous carbons is through the use of mesoporous inorganic solid templates(8) (steps (i) and (ii)). By adsorption of a source of carbon such as sucrose into the pores (step (iii)) followed by decomposition of the carbon source (step (iv)) and dissolution of the inorganic template (step (v)), a mesoporous carbon can be formed. However, the method is multi-step, energy-intensive, and wasteful.
Another approach to the preparation of mesoporous carbons is through metal carbide precursors, for example zirconium carbide. However, these require the preparation of the precursors, can be expensive and only in some cases give a mesoporous structure.
Starch and cellulose are biopolymers produced by plants. They are non-toxic, naturally abundant and biodegradable and as such represent a vital renewable resource for sustainable development. Like all organic materials, they can be carbonised, typically by heating to high (>300° C.) temperatures in air. The carbons produced in this way from ordinary native (i.e. non-modified) cellulose and starches are normally of limited value due to high microporosity and very little control is possible in the preparation over the bulk or surface structure. There is a need to develop new, simpler and less wasteful routes to mesoporous carbons and a need to design new forms of carbon especially with controlled bulk and surface structures, functionalisation (such as acidity) or derivatisation (such as metal adsorption) and activity in aqueous environments.
The terms “mesoporous”, “mesoporosity”, “microporous” and “microporosity are used herein in accordance with IUPAC (International Union of Pure and Applied Chemistry) standards. Mesoporosity includes pore size distributions typically between 2 to 50 nm (20 to 500 Å) whereas materials with pore sizes typically smaller than 2 nm (20 Å) are considered as microporous.